The present invention relates to a method of stabilizing the surface properties of objects which are subjected to a vacuum temperature process.
Such a process step is used, for example, in the manufacture of pressure sensors. U.S. Pat. No. 50 50 034, for example, describes the manufacture of a capacitive pressure sensor having a substrate and diaphragm which are to be joined together, particularly in a defined spaced relationship and parallel to each other, forming a chamber sealed at least at the edge, the substrate and/or the diaphragm being made of ceramic, glass or a single-crystal insulating material. This method comprises the following steps. The diaphragm is covered with a layer of silicon carbide, niobium or tantalum which serves as one of the capacitor electrodes; a portion of the substrate surface which will lie within the chamber is covered with at least one additional layer of any of the aforementioned conductive materials which serves as the second etc. capacitor electrode; contact is made to the capacitor electrodes through the substrate, and substrate and diaphragm are high-vacuum-brazed together by means of a ring-shaped part of active brazing material, which also serves as a spacer, or by means of an amount of active brazing paste sufficient for holding the two parts at the desired distance from each other. The ring-shaped part is spaced apart from the second capacitor electrode formed on the substrate surface to electrically isolate the first and second capacitor electrodes.
Pressure sensors thus manufactured are extremely moisture-sensitive, which greatly reduces the Q of the capacitor(s) in particular. Investigations have shown that this moisture sensitivity is not due to changes in the electrodes or the strain-gage material during the high-vacuum brazing, but that in this process step the uncovered surface portions of substrate and diaphragm change so that they, instead of retaining their very good insulating ability, become semiconducting and highly moisture-sensitive.
For example, an experimental capacitive reference pressure sensor with a 60-pF precision capacitor and a 60-pF reference capacitor exhibited, at zero pressure, a capacitance difference of 1.5 pF at a tan-delta difference of 0.05 and a reference-capacitance difference of 3 pF at a tan-delta difference of 0.1 for a change in relative humidity from 30% to 85% (at a temperature of 20.degree. C.).
Furthermore, prior to the high-vacuum brazing, an experimental substrate of alumina ceramic with 96% purity on which two concentric coatings are deposited to form said second capacitor electrode of the above precision and reference capacitors were deposited at a distance of 1 mm from each other, with the inner electrode having a diameter of 16 mm, showed a resistance, measured between these two electrodes, of 4.times.10.sup.13 ohms in a dry atmosphere (=0% relative humidity), but a resistance of 1.times.10.sup.11 ohms at 70% relative humidity (again at 20.degree. C.). After the high-vacuum brazing, the corresponding resistance values were 3.times.10.sup.13 ohms for a dry atmosphere and only 3.times.10.sup.8 ohms for 70% relative humidity.
These investigations led to the recognition that as a result of the high-vacuum brazing, those surface portions of the substrate and diaphragm which will not be covered within the chamber to be formed lose oxygen or nitrogen atoms, i.e., that these surface portions are reduced. This results in these portions becoming semiconducting, for example, which causes the above-mentioned degradation of the Q of the capacitors and the changes in capacitance.
Thus, as a rule, vacuum temperature processes do not leave the surface properties of the treated objects unaffected and mostly deteriorate them.
The invention serves to solve this problem.